1. Field of the Invention
The invention relates to a semiconductor unit and a method for mounting a semiconductor device, and particularly to a method for forming a bump for electrically connecting a terminal electrode of a circuit board with an electrode pad of a semiconductor chip, to a method for mounting a semiconductor device using the bump, and to a semiconductor unit using the bump.
2. Description of the Related Art
Conventionally, a wire bonding method using a soldering technique has been widely used in a process for mounting a semiconductor device on a circuit board having input/output terminal electrodes. Recently, however, because the size of the package for a semiconductor device has been reduced and the number of connecting terminals has been increased, the pitch of the connecting terminals has been shortened, thereby making conventional soldering methods difficult to use for such a bonding application.
In order to solve this problem, recently, improved methods have been proposed in which a semiconductor device such as an integrated circuit chip, is directly mounted on input/output terminal electrodes of a circuit board, so that the mounting area is reduced in size, thereby improving the utility efficiency.
Among these improved methods, a flip chip bonding method in which a semiconductor device is mounted facedown on a circuit board is known to be effective because the semiconductor device is electrically connected to a large number of the terminal electrodes of the circuit board at a time with the resulting connections having excellent mechanical strength.
For example, a mounting method using a ball bonding technique is described in "IC mounting technique", edited by Nippon Microelectronics Association, KOGYO CHOSAKAI, Jan. 15, 1980. Hereinafter, this mounting method will be described.
When an electrode pad 12 of a semiconductor device (IC substrate) 11 is to be connected with an input/output terminal electrode 18 of a circuit board 19, an adhesion metal film 22 and a diffusion-preventing metal film 21 are first formed on the electrode pad 12 of the semiconductor device 11 by a vapor deposition technique as shown in FIG. 6A, and an electrical contact (projecting electrode: bump) 20 made of solder is then formed by a plating technique. Thereafter, the thus formed IC chip is positioned so that the solder bump 20 contacts with the terminal electrode 18, and then placed facedown on the circuit board 19 as shown in FIG. 6B. The semiconductor unit is then heated to a high temperature so that the solder bump 20 is fused and bonded to the terminal electrode 18 of the circuit board 19.
Recently, as shown in FIG. 7, a semiconductor unit is proposed in which an Au bump 30 is formed on an electrode pad 32 of a semiconductor device 31 by a plating technique and the Au bump 30 is connected with a terminal electrode 38 of a circuit board 39 through a conductive adhesive 37. In such a semiconductor unit, after the conductive adhesive 37 is transferred to the bump 30 of the semiconductor device 31, the positioning process is conducted in such a manner that the bump 30 contacts with the input/output terminal electrode 38 of the circuit board 39, and the conductive adhesive 37 is then cured to obtain an electrical connection.
When pitches of terminal electrodes are to be formed more finely, however, there arise the following problems in such a bump for a conventional semiconductor device, a semiconductor device or a semiconductor unit using such a bump.
1. When the solder bump 20 is heated to a high temperature (200 to 300.degree. C.) to be fused and connected with the input/output terminal electrode 18, it is impossible to maintain the separation distance between the metal film on the surface of the IC substrate 11 and the terminal electrode 18, resulting in a danger that the fused solder spreads to reach an adjacent electrical contact or bump, thereby causing a shortcircuit.
2. The electrical contact (bump) 30 has a section shape of a trapezoid (straight wall type) or a semicircle (mushroom type). When the bump 30 is connected with the input/output terminal electrode 38 through the conductive adhesive 37, there is a risk that the conductive adhesive 37 spreads to reach an adjacent contact or bump, thereby causing a shortcircuit.
3. When the bump 30 is connected with the input/output terminal electrode 38 through the conductive adhesive 37, the front end face of the bump 30 to which the conductive adhesive 37 has been transferred is pressed with a load against the terminal electrode 38 on the circuit board 39. Therefore, the most part of the conductive adhesive 37 is formed so as to surround the bump of the bonding portion (this surrounding part is referred to as a bulk portion). Consequently, when a stress due to thermal expansion caused by a thermal shock, or a reduction of the adhesion strength produces a crack or separation in the bulk portion, there is a risk that the bonding interface is made unstable so that the resistance of the electrical contact is increased.
The invention has been directed in order to solve the above-mentioned problems. It is an object of the invention to provide a method for forming a projecting electrode for a semiconductor device, a method for mounting a semiconductor device, and a semiconductor unit which enable an electrical connection with a fine pitch between the semiconductor device and a circuit board to be conducted easily and surely.